Wave-cut or water-table platforms of rocky coasts and rivers?

[摘要] Despite evidence that weathering plays a role in forming intertidal,estuarine, and fluvial rock platforms, many publications uncritically refer to “wave-cut”or “stream-cut” rock platforms. The alternative explanations of chemical weatheringand physical water erosion of rock platforms have been debated since 1839. Our newapproach to this long-standing problem introduces data from fluvial as well as coastalrock platforms and documents the saprock-bedrock division of weathering profiles.We report local field evidence for a dominant role of differential weathering ofsedimentary beds in an intertidal rock platform at Sunset Bay and a riverside platformnear Days Creek, both in southwest Oregon, USA. Despite continuing tectonic uplift,both rock platforms remain within the zone of modern water table and intertidalfluctuation. Above this zone of frequent wet/dry cycling, rock is weakened by subaerialweathering, as documented by decreased rock hardness from the application of a Schmidthammer, high slaking quantified by wetting-drying experiments, Munsell color indicationof elevated oxidation of fine fractures, and peripheral loosening of calcareousfossils. At these sites, the shape and elevation of rock platforms in coastal andfluvial settings appear to reflect differences in strength between bedrock and saprockwithin the zone of water table fluctuation. In our study sites, waves and floodsdo not “cut” rock but remove clasts already weakened by weathering to expose thelocal water table as a bedrock surface. Other local agents of rock removal includesalt weathering, eolian abrasion, landslides, debris flows, and fungal, plant, andmolluscan bioerosion.

[学科分类] 地质学

[关键词] Fire safety;fatal fires;smoke alarms